1. Field of the Invention
The invention relates to an information system comprising a record carrier on which information has been recorded in the form of variations of a first physical parameter and a playback apparatus provided with means for scanning the record carrier by means of a transducer which is responsive to said variations of said first physical parameter, and means for recovering the information from a detection signal received from said transducer.
The invention further relates to a record carrier and a play-back apparatus for use in the system.
2. Description of the Related Art
A system of the type mentioned above is inter alia known as the Compact-Disc system. Normal Compact Discs play on all compatible playback apparatuses. Nowadays recording apparatus are available for copying the information present on a read-only Compact Disc on a recordable disc which can be played on the compatible play-back apparatus.
However some applications , for example audio-visual games, require a so-called xe2x80x9cclosed systemxe2x80x9d in which the Compact Disc with the software representing the audio-visual game can only be played on special players and which cannot be copied easily by means of the available recording apparatuses.
In view of cost price aspects it is desired that know-how of the already existing information systems can be used as much as possible. Therefor it is desired to modify existing information systems as little as possible in order to realize a low-cost xe2x80x9cclosed information systemxe2x80x9d.
However it should be made very difficult for third parties to copy such special discs by means of existing copying machines.
The prior art protection schemes do not meet this requirement, e.g.
rate scrambling/encryption can by copied from disc to disc with a bit copying machine;
special logical errors (to copy protect) in a main and/or subcode channel (of e.g. a Compact Disc) can also be copied with a bit copying machine;
schemes relying on the relation main/subcode channel can also be copied with a bit copying machine.
An object of the invention is to provide a closed information system in which record carriers are used which cannot easily be copied.
According to the invention this object is achieved by an information system as described in the opening paragraph and which is characterized in that said record carrier exhibits a second variation of a second physical parameter, which differs from said first physical parameter but is detectable by said transducer, the playing apparatus comprises detection means for detection of the presence of said second variation on the basis of a detection signal received from the transducer, and means responsive to the detection means for enabling recovery of the information in the event that the presence of said second variations is detected.
Due to the fact that a bit copying machine usually only copies the variations of the first physical parameter (which variations represent the information recorded) the variations in the second physical parameter are not copied. Consequently the special discs cannot be copied by the usual type of bit copying machines.
An embodiment of the information system is characterized in that the second variation exhibits a modulation pattern representing a code, the detection means comprises code recovery means for recovering the said code from the detection signal and also comprises means for activating the enabling means in response to the recovery of said code.
The use of a modulation of the variation has the advantage that the presence of the variation of the second physical parameter can be detected more reliably.
A further embodiment of the information system is characterized in that the information recorded is of a type which is recoverable by means of a predetermined data processing, the code represented by the modulation pattern of the second variation indicates the type of data processing to be used for recovering the information, and the apparatus is provided with means for setting the recovery means in a mode in which the recovered predetermined data processing indicated by the code recovered is performed.
This embodiment has the advantage that for recovering the information read from the record carrier it is required that the code represented by the modulation pattern is available. So the information can only be recovered by a dedicated play back apparatus, which is able to recover the code. In the event that the information is encrypted or scrambled before it is recorded on the record carrier the code preferably indicate the encryption key or the scramble method respectively.
Although not limited to information systems in which optically readable record carriers are used, the system is in particular suitable for such information systems.
In an optical record carrier it is relatively simple to provide the track in which the information has been recorded with a further track modulation which can be detected by the same radiation beam as used for reading the information.
An embodiment of the information system in which this is realized, is characterized by servo control means for controlling the scanning in order to control at least one scanning parameter to a predetermined value on the basis of a detection signal received from the radiation sensitive detector and which is affected by said second physical parameter, which servo control means has a predetermined frequency bandwidth, the variation of said second physical parameter causes variations in the detection signal which exhibit a frequency spectrum which is located outside the bandwidth of servo control means and outside the frequency spectrum of the signal variations caused by the variation of the first physical parameter.
The variations in the second physical parameter can be in the form of a variation in the track position in a direction transverse to the track direction. This variation can be detected on the basis of the tracking error signal.
The variations in the second physical parameter can be in the form of a variation in the position of the plane in which the optically readable marks are located. In that event the variation can be detected on the basis of the focus error signal.
The variations in the second physical parameter can also be in the form of a variation in the mean value of the optically readable marks and the intermediate areas located between the optically readable marks. In that event the variations in the second physical parameter can be detected on the basis of variations in the data clock signal which is recovered during the scanning of the track with a constant linear velocity.
In the event that the record carrier used in the information system is a Compact Disc it is preferred to use an information system which is characterized in that the variations in the second physical parameter result in a variation in the detection signal with a frequency substantially corresponding to 22 kHz in the event that the track is scanned with a scanning speed between 1.2 to 1.4 meter per second.
This embodiment has the advantage that it is impossible to copy the special disc on the usual recordable Compact Disc, which has a pregroove which exhibits a wobble which results in a tracking error with a frequency of substantial 22 kHz when the pregroove is scanned with a velocity between 1.2 and 1.4 meter per second.
Even in the event that it is possible to record a wobbling pattern of recording marks which corresponds to the wobbling recording marks of the record carrier to be copied, this pattern will not be detectable because of the presence of the wobbling pregroove which is situated in the same frequency range.